The present invention is directed toward a wavelength division multiplexed (WDM) ring system having reduced adjacent channel cross-talk.
Optical communication systems are a substantial and fast growing constituent of communication networks. The expression "optical communication system," as used herein, relates to any system which uses optical signals to convey information across an optical waveguiding medium, for example, an optical fiber. Such optical systems include but are not limited to telecommunication systems, cable television systems, and data communication networks, such as local area networks (LANs) and wide area networks (WANs).
Currently, many optical communication systems are configured to carry an optical signal of a single wavelength over one or more optical waveguides such as optical fibers. To convey information from multiple sources, time-division multiplexing (TDM) is frequently employed. In TDM, a particular time slot is assigned to each signal source with the complete signal constructed from portions of the signal collected from each time slot. While this is a useful technique for carrying plural information sources on a single channel, i.e., at a single wavelength, it is prohibitively expensive to increase the TDM data rate.
While the need for communication services increases, the current capacity of existing waveguiding media is limited. Although capacity may be expanded (e.g., by laying more fiber optic cables), the cost of such expansion is often prohibitive. Consequently, there exists a need for a cost-effective way to increase the capacity of existing optical waveguides.
Wavelength division multiplexing (WDM) is an approach for increasing the capacity of existing fiber optic networks. WDM systems typically include a plurality of transmitters, each respectively transmitting signals on a designated wavelength. As a result, fiber capacity can be increased by a multiple equal to the number of wavelengths or channels.
WDM systems have been deployed in long distance networks in a point-to-point configuration consisting of end terminals spaced from each other by one or more segments of optical fiber. In metropolitan areas, however, WDM systems having a ring or loop configuration are currently being developed. Such systems typically include a plurality of nodes located along the ring. At least one optical add/drop element, associated with each node, is provided along the ring to permit both addition and extraction of optical signals at a particular wavelength to and from the ring. One of the nodes, referred to as a hub or central office node, has a plurality of associated add/drop elements for transmitting and receiving a corresponding plurality of optical signals at respective wavelengths to/from other nodes along the ring.
Each optical signal in a WDM system is typically at a wavelength within a relatively narrow range about 1550 nm, which is the absorption minimum associated with most silica-based optical fibers. Accordingly, the wavelengths are somewhat narrowly spaced, typically by about 100-200 GHz, but sufficiently far apart to be separated by add/drop elements including dielectric filters. The filters, however, still drop an attenuated portion of optical signals at wavelengths close to the desired wavelength. Typically, provided that the power level of an optical signal at the adjacent wavelength is not significantly more than the power level of the optical signal at the desired wavelength, the filter can output the desired optical signal at a level at least 20 dB greater than the optical signal at the adjacent wavelength power level, thereby permitting accurate detection of the desired optical signal.
In a WDM ring system, however, the optical signal at the desired wavelength may be transmitted from an emitter located at a node spaced relatively far from the corresponding receiver, while an emitter transmitting an optical signal at a wavelength adjacent the desired wavelength may be spaced relatively close to the receiver sensing the optical signal at the desired wavelength. As a result, the power level of the optical signal at the adjacent wavelength input to the filter at the receiver can be significantly greater than that of the optical signal at the desired wavelength. Thus, both optical signals at the desired and adjacent wavelengths are supplied to the receiver at comparable power levels. Such "adjacent channel cross-talk" prevents accurate detection of the optical signal at the desired wavelength.
Moreover, each filter imposes an incremental loss on optical signals propagating along the WDM ring. Accordingly, a particular optical signal traversing a given number of filters along the ring can incur significantly more loss than other optical signals at different wavelengths traversing fewer filters. The loss associated with the particular optical signal can be so high as to limit the ring circumference.